Bladed aircraft rotor with flexible blade mountings

ABSTRACT

A two-bladed aircraft rotor is disclosed having blades mounted for freedom of blade motions, at least in the flapping sense and in the pitch-change sense, with the blades driven by flexible bellows.

This is a divisional application of my co-pending application Ser. No.818,938 filed Jan. 15, 1986, now U.S. Pat. No. 4,708,591 dated Nov. 24,1987 for BLADED AIRCRAFT ROTOR WITH FLEXIBLE BLADE MOUNTINGS

BACKGROUND AND STATEMENT OF OBJECTS

This invention relates to bladed aircraft rotors, for example bladedhelicopter sustaining rotors, and while various aspects of the inventionare adapted to and usable in other bladed rotors, such as a so-calledtail rotor of a helicopter for controlling the aircraft in yaw, theinvention is herein described and illustrated as embodied in ahelicopter sustaining rotor.

In conventional aircraft sustaining rotor systems, particularlypower-driven helicopter rotor systems, it has been customary to employrotor blade mounting pivots, usually including at least two pivots foreach blade, i.e., a blade flapping pivot, and a pitch change pivot.So-called drag pivots are also quite commonly employed. The blade pivotshave been utilized extensively for the purpose of providing freedom forthe required blade motions, such as the flapping motion, the pitchchange motion, and the lead-lag motions of the blades, those variousmotions being desirable in various flight operations, particularly incontrolled maneuvering and translational flight. While the blade pivotstructures referred to have served those purposes, the employment ofsuch pivots involves expensive manufacturing costs and requires costlymaintenance, and in addition, represents potential safety hazards in theoperation of such aircraft.

In order to eliminate the disadvantages incident to the employment ofblade mounting pivots, certain flexible blade mounting arrangements havebeen proposed, as disclosed, for example, in my prior U.S. Pat. Nos.4,266,912, issued May 12, 1981, and 4,502,840, issued Mar. 5, 1985, thedisclosures of which are incorporated herein by reference.

In the '912 patent, there are disclosed flexible straps (herein referredto as flex straps) employed for connecting the root ends of the rotorblades to a hub structure, the straps providing freedom for variousblade motions with respect to the hub structure, including motions inthe flapping plane, lead-lag motions, and pitch change motions. In the'840 patent, flex straps for mounting the blades are also disclosed,together with controllable pitch change mechanism.

The present invention contemplates employment of flex strap arrangementsand pitch control mechanisms of the kind disclosed in my prior patentsabove-identified, but in addition, the present application is concernedwith a number of improvements in rotors of this type in which blademounting pivots are not used.

In considering one aspect of the present invention, it is first notedthat the required flapping motions of the rotor blades representssubstantial motions in the flapping plane, such flapping motionsoriginating from various sources, including differential liftcompensation at opposite sides of the rotor in translational flight, andalso extensive motions in the flapping plane resulting from impositionof the cyclic pitch control, with resultant change in attitude of theaircraft, which frequently requires quite large additional flappingmotions superimposed upon those resulting from differential liftcompensation in translational flight.

With the foregoing in mind, the present invention contemplates theemployment not only of the flex straps above referred to, but also of aflexible joint at the center of the rotor or rotor hub mechanism, whichflexible joint will accommodate conjoint equal and opposite flappingmotions of the blades such as are frequently encountered in maneuveringcontrol of the aircraft. By the employment of this central flexiblejoint, the extent of flapping motions required to be accommodated by theflex straps is greatly reduced, so that, in effect, the overall flappingmotions are divided and distributed between two different mechanisms,instead of relying solely upon the flex straps for all of the flappingmotion accommodation

For the above purpose, the invention contemplates employment of aspherical center joint through which the blades are connected with acentral mounting structure, for instance, the rotor drive shaft, and aswill be explained more fully hereinafter, the form of the centralflexible joint employed not only accommodates and absorbs a portion ofthe flapping motions of the blades, but also is resistant to deflectionin a direction parallel to the axis of rotation, and, therefore, servesas the load transmitting component between the body of the aircraft andthe sustaining blades.

The foregoing accommodation of different portions of the flappingmotions by different mechanisms is of importance in connection with theutilization of the flex straps, because the reduction in the overallflexibility requirements of the flex straps makes possible theemployment of a wider range of materials in the fabrication of thestraps themselves.

In addition, the employment of the central spherical flexible jointaccommodating a portion of the flapping motions of the blades results inthe concentration and handling of the equal and opposite flappingmotions of the blades at opposite sides of the rotor about the centerpoint of the central joint. Since this center point lies on the axis ofrotation of the rotor, this diminishes the Coriolis effect which wouldotherwise be set up as a result of the total flapping action beingabsorbed by the flex straps, as will be explained more fullyhereinafter.

In accordance with another aspect of the present invention, a novel formof torque transmission mechanism is employed between the drive shaft forthe rotor and the rotative parts of the hub structure, this drivetransmission comprising flexible sheet material in the form of bellowsmounted above and below the plane of connection of the rotor blades tothe central flexible joint. Edges of the bellows are respectivelyconnected with the drive shaft and the blade mounting parts of therotor, thereby providing a constant torque drive system, even when theblades occupy equal and opposite flapping positions at opposite sides ofthe rotor. By the employment of bellows both above and below the planeof the blade mounting, the flapping motions of the blades provided bythe central joint are not only accommodated, but in addition, the torqueloads are divided between the upper and lower bellows, thereby makingpossible the employment of economical flexible sheet material in theformation of the bellows.

BRIEF DESCRIPTION OF THE DRAWINGS

How the foregoing objects and advantages are obtained will appear morefully in referring to the accompanying drawings in which:

FIG. 1 is an outline side elevational view of a helicopter embodying theimprovements provided according to the present invention;

FIG. 2 is an enlarged fragmentary plan view of the central hub structureand portions of the root end mounting parts for the blades of afour-bladed rotor;

FIG. 3 is a vertical sectional view through the hub structure employedin FIGS. 1 and 2, including the root end blade mounting arrangement, thecentral flexible joint, and the driving bellows above and below theplane of blade attachment;

FIG. 4 is an enlarged fragmentary sectional view of a portion of a jointpreferably employed at the center of the hub structure;

FIG. 5 is a fragmentary view taken substantially as indicated by thesection line 5--5 in FIG. 3, illustrating the inclination of the blademounting fittings;

FIG. 6 is a view similar to FIG. 3 but illustrating an inclined ortilted position of the rotor as a whole;

FIG. 7 is a vertically exploded view of portions of the rotor shaft andof the blade mounting parts associated with that shaft;

FIG. 8 is a view similar to FIG. 3 but illustrating a modified form ofrotor system having two blades, instead of four blades shown in theembodiment of FIGS. 1 to 7 inclusive, this view illustrating theemployment of the torque transmitting bellows above and below the rotorin a two-bladed rotor system in which the two blades are mounted bymeans of a horizontal pivot intersecting the axis of rotation;

FIG. 8a is a fragmentary detailed plan view on the line 8a-8a on FIG. 8;

FIG. 9 is a view similar to FIG. 8 but illustrating an embodiment inwhich the central pivot for a two-bladed rotor, as shown in FIG. 8, isreplaced by a multi-layer cylindrical flexible joint;

FIG. 10 is a fragmentary perspective view of portions of the centerjoint shown in FIG. 9 and

FIGS. 11A and 11B are somewhat diagrammatic side elevational views oftwo forms of rotors in translational flight one rotor (in FIG. 11B)being of the form having a center flexible joint of the kind shown inFIGS. 1 to 10 inclusive, and the other rotor (in FIG. 11A) being of aform in which the center joint is not included, and illustrating acomparison between the blade positions in the two forms.

DETAILED DESCRIPTION OF THE DRAWINGS

In a helicopter such as shown in FIG. 1, the sustaining rotor is mountedfor rotation about an upright axis above a body structure such asindicated at 11, the sustaining rotor comprising a plurality of blades12 extending radially from the central hub structure indicated ingeneral at 13. While any of a wide variety of drive systems may beincluded in such a helicopter FIG. 1 indicates a driving engine 14mounted on an upright axis, the drive system preferably including adisconnectable clutch, for instance, in the region indicated at 15,which may also embody an overrunning clutch so that in the event ofengine failure, the rotor may be turned autorotatively. A gearinginterconnection 16 may be provided between the rotor drive shaft and atail rotor such as indicated generally at 17, as commonly provided onhelicopters for the purpose of yaw control. A drive shaft 18 may extendfrom the gear box 16 to the tail rotor 17, and the tail rotor may beprovided with appropriate control mechanism, such as blade pitchcontrol, in order to counteract rotor driving torque and also to steerthe aircraft in yaw.

As will be understood from FIG. 2, the rotor of this embodiment includesfour blades, two of which appear at 12. Each blade has a flex strap 19at the inner end of the blade, the straps being connected with theforked blade mounting fittings 20, four such fittings being provided atthe periphery of a mounting ring for the blades, herein referred to as ablade retention ring, this ring being shown at 21 in FIGS. 3, 5 and 6.

As is brought out in my prior patents above-identified, the flex straps19 employed in the rotor system herein disclosed are adapted to providefor blade motions in several different senses. Thus, the flex straps areconstructed to provide freedom for flapping motion of the rotor blades,i.e., motion of the blades in a generally vertical direction transverseto the mean plane of rotation thereof. In addition, such flex straps mayalso provide for pivotal motion of the blades in the plane of rotation,i.e., in the lead-lag sense.

Still further, the flex straps may be relied upon to provide freedom forpitch change motion of the blades, i.e., change in the pitch angle ofthe blades under the control of a pitch control system as disclosed, forexample, in my prior U.S. Pat. No. 4,502,840 above referred to. Thisprovision for pitch change of the blades provides for employment ofrotor control by cyclic pitch change in known manner, providing forinclination of the plane of rotation of the rotor in any desireddirection, for instance, in the forward direction for establishingtranslational flight. As will be seen in FIG. 2, a fitting 22 isconnected with each blade between the flex strap and the blade itself,this fitting having a rearwardly projecting lug 23 providing forconnection with the pitch control rod 24 which extends inwardly towardthe rotor hub and which is journaled adjacent to the hub as indicated at25. A pitch control arm 26 is secured to each of the pitch control rods24, this pitch control arm serving as a mean for turning the rod 24 andthus turning the fitting 22 connected with the root end of the blade,this, in turn, resulting in change in the pitch angle of the blade, asprovided for by the torsional flexibility of the flex strap 19.

The pitch arms 26 may be connected by means of links 27 extendingdownwardly for connection with the rotative portion of the cyclic pitchcontrol swash plate or other similar mechanism providing for the desiredcyclic pitch control of the rotor blades. The pitch control mechanismpreferably also provides for conjoint controlled vertical movement ofthe pitch control links 27, thereby providing for collective pitchchange of all of the blades in the same sense, as is required undervarious operating conditions, including vertical ascent and descent.This collective pitch control may also provide for a mean pitch settingsuitable for autorotative operation of the rotor in the event of enginefailure.

As seen in FIG. 2, a streamlined sheath or enclosure 28 may be providedin the region to the rear of the flex strap 19 of each blade andenclosing the pitch rod 24. This streamlined enclosure may be secured tothe lugs 23 and, therefore, moveable therewith during the pitch controloperations.

Turning now to the illustration of the rotor hub structure as appears inFIGS. 3 to 7 inclusive, it will be seen that each of the forked blademounting fittings 20 projects radially outwardly from the bladeretention ring 21. The blade retention ring 21, in turn, is mounted on acentral shaft indicated at 29 comprising an inner rotative part of thehub structure, which also includes the outer tubular shaft 30 which isdriven by the engine 14 shown in FIG. 1 through the appropriate gearingand clutch connections.

The blade retention ring 21, in accordance with the present invention,is connected with the central shaft 29 by means of an elastomeric jointincluding various of the components now to be described. Thus, as seenin FIGS. 3, 6 and 7, the central shaft 29 is connected at its upper endwith a disc part 31 lying in a plane transverse to the axis of the shaft29. A disc part 32, having a central sleeve 33, is removably secured tothe lower end of the shaft 29 by means of the threaded ring 34.

At the lower side of the upper disc part 31, there is provided anannular abutment 35 having an abutment surface concavely and sphericallycurved about a center point x (see particularly FIGS. 3 and 6) on theaxis of rotation in the mean plane of the blade retention ring 21.Similar1y, an annular abutment 36 is mounted on the upper side of thedisc part 32 having an abutment surface concavely and spherically curvedabout the center point x on the axis of rotation in the mean plane ofthe blade retention ring 21.

The blade retention ring 21 is provided with upper and lower abutmentdevices 37 and 38, these abutment devices having convexly sphericallycurved abutment surfaces spaced from but concentric with the surfaces onthe abutment devices 35 and 36, and thus also centered on the centerpoint x. An elastomeric joint structure generally indicated at 39--39intervenes between the concavely curved and convexly curved surfaces ofthe abutment devices 35-37 and 36-38. This elastomeric joint may beformed of a variety of materials, but preferably comprises multiplealternate spherical layers of elastomeric and metallic materials bondedto each other. The elastomeric layers provide for yielding relativemotions of the intervening metallic layers and, in this way, providefreedom for restrained tilting motions about the center point x, whileresisting axial motions in a direction coincident with the upright axisof rotation of the rotor. The axial thrust or lift of the rotor is thustransmitted to the mounting or drive shaft 30 for the rotor. This jointpreferably also prevents lateral shift of the blade retention ring inthe plane of rotation of the blades, i.e., in directions transverse tothe axis of rotation. FIG. 4 is a fragmentary illustration of thealternating spherical layers employed in the joint 39, i.e., themetallic layers 40 and the rubber or other elastomeric layers 41.

This joint 39 thus provides freedom for tilting motion of the bladeretention ring 21 in any direction, but, at the same time, substantiallyinhibits displacement motion of the blade retention ring 21 either in adirection in the plane of rotation of the rotor or in a directionparallelling the axis of rotation of the rotor. The central joint 39thus contributes freedom for angular motion of the blades, particularlyin the flapping plane, and thereby provides freedom for tilting orinclination of the mean plane of rotation of the rotor in any directionabout the central point x. In consequence of this freedom, when theplane or rotation is inclined downwardly at one side of the rotor andinclined upwardly at the other side of the rotor, for instance, intranslational flight when the plane of rotation of the rotor is tilteddownwardly in the front and upwardly at the rear, this tilting of therotor blades can be accommodated by the central joint 39 and need notrequire reliance upon flexibility of the flex straps 19. The flexibilityof the flex straps in the flapping plane, therefore, need not be asgreat as wou1d be the case where the central elastomeric joint 39 is notprovided, and this is of advantage from the standpoint of wear ofmaterials and also from the standpoint of the flexibility and othercharacteristics of the materials to be employed in the fabrication ofthe flex straps.

In connection with the mounting of the rotor blades on the bladeretention ring 21, attention is directed to FIGS. 3 and 5 whichillustrate preferred preset blade angles. It will be noted that thefittings 20 are positioned to receive the inner ends of the flex strapsat a somewhat upwardly coned angle, for instance, an angle (a) of theorder of 3 or 4 degrees above the horizontal, thereby establishing apre-cone angle for the rotor blades from which the blade flapping actionwould extend above and below, in accordance with the various forcesinfluencing the flapping action. In addition, it is preferred that theblade mounting fittings be arranged to pre-establish a pitch angle (θ)of the rotor blade of the order of about 13 to 15 degrees, as isindicated in FIG. 5. In this way, the normal cyclic and collective pitchranges would represent approximately equal deflections from thepre-established angles for all normal flight conditions.

It will be seen that in the absence of some other torque connectionbetween the blade retention ring and the rotor drive shaft 29-30, theelastomeric joint 39--39 between the blade retention ring and thecentral rotor shaft would have to carry the rotor driving load. However,it is contemplated according to the invention that provision be made fortorque or driving interconnection independent of the elastomeric joint39--39, thereby relieving the elastomeric joint 39--39 of anysubstantial rotative torque forces For this purpose, the presentinvention provides a novel drive system, herein referred to as a "soft"hub, interconnecting the driven shaft 30 of the rotor hub structure andthe blade retention ring 21. As seen in FIGS. 3, 6, and 7, this torquetransmissions mechanism comprises a pair of flexible bellows 42--42, onepositioned above the blade retention ring 21 and the other below theblade retention ring 21. Both of these bellows are secured, as by bolts43, to the blade retention ring. The upper bellows is connected by bolts44 with the peripheral portion of the disc part 31. The lower bellows isconnected by bolts 45 with the disc part 32, this latter disc part alsobeing secured by bolts 46 to the annular ring 47 which is connected orintegral with the upper end of the rotor drive shaft 30.

The bellows 42--42 are formed of sheet-like material which istransversely flexible but which is resistive to deflection in the planeof sheet material. Materials suitable for this purpose may comprisevarious materials having reinforcement fibers embedded therein. Forexample, appropriate materials for this purpose are resin materials suchas melamine formaldehyde and phenol formaldehyde resins, together withreinforcing fibers, for example, boron, graphite, fiberglass andpolyamide fibers, such as the aramid polymers. For various purposes, itmay also be desirable to employ combinations of fibers and still furtherto employ such combinations of fibers in resin materials of varioustypes, according to the requirements of the particular installation.Further commercial examples are Narmco 5225 Celion 12K; AmericanCyanimide 5143 and 5143LRS; and Ferro E293FC and Narmco 5225.

Comparison of FIGS. 3 and 6 will show the deflection of the bellowsaccompanying tilting movement of the blade retention ring 21 about thecentral pivot point x.

The arrangement of the torque transmitting bellows, as described abovein connection with FIGS. 3, 6, and 7, is particularly advantageous froma number of standpoints, including the fact that the subdivision of thebellows into two parts, one arranged above the plane of rotation and theother below the plane of rotation of the blade retention ring, enablesthe employment of more flexible torque transmitting sheet material whilemaintaining the thickness of the material at a value which will nothinder the tilting motion of the blade retention ring about the centerpoint x.

Such a driving bellows arrangement is also adaptable to other forms ofbladed rotors, including, for example, the forms of rotors shown inFIGS. 8 and 9.

In FIG. 8, the rotor illustrated comprises only two rotor blades whichmay be connected with the forked fittings 20a which are provided onindividual blade mounting links 48 which are fastened, as by bolts 49,with a central mounting piece 50 which is journaled on the rotor shaft51a by means of a single central pivot 52, having surrounding roller orneedle bearings, the center of this pivot intersecting the rotor axis,so that the blades connected with the forked fittings 20a may rock aboutthe axis of the central pivot 52. It will be understood that in FIG. 8,the two blades here employed may be arranged in the general mannerillustrated in FIG. 2 with respect to the employment of the flex straps19 and the cyclic pitch control system.

In the embodiment of FIG. 8, the central shaft 51a is provided withlaterally spaced flat or rectangular portions 51b providing a centraltransverse channel through which the central mounting piece 50 for thetwo rotor blades may extend, and the central pivot 52 may extend throughand be secured to the portions 51b, thereby providing freedom forrocking movement of the mounting piece 50 with respect to the rotorshaft 51a. As in the embodiment shown in FIGS. 3 to 7, the central rotorshaft 51a is connected with the outer shaft 30 by means of a disc part32 and an annular ring 47, these parts being connected with each otherby means of bolts 46.

As in the embodiment described above, upper and lower flexible torquebellows 42 are also provided and are connected with the upper and lowerparts 31 and 32 of the rotor shaft by means of bolts 44 and 45. Theadjacent edges of the upper and lower bellows are also fastened to theblade mounting links 48 by means of bolts 43.

This torque or driving bellows arrangement is closely similar to thatdescribed above in connection with the first embodiment, but it is herenoted that in the embodiment of FIG. 8, the central joint comprises aroller or needle bearing structure 52a surrounding the central pivot 52.With a construction of this type, it is preferred to provide someclearance for the common teetering pivot with respect to the centralrotor shaft 51a i.e., a freedom for some movement of the blades and themounting pivot in a rotational sense about the axis of the rotor, sothat rotor torque, particularly under rotor driving conditions, will beassumed by the flexible bellows 42 and will not impose appreciabletorque force through the central teetering pivot. In this arrangement,it is also contemplated that, notwithstanding freedom provided fortorsional movement of the blades with respect to the hub, the centralpin 52 should also serve for transmission of rotor thrust from thecentral blade mounting piece 50 to the rotor shaft 51a.

With the foregoing in mind, it is contemplated to employ a sphericaltype of roller bearing for the central bearing structure 52a, asillustrated in FIGS. 8 and 8a. This provides for adjustment movement ofthe blade mounting piece 50 about the axis of the rotor thereby assuringthat the rotor torque will be carried by the bellows 42--42 when therotor is being driven.

In the embodiment of FIGS. 9 and 10, a teetering type of mounting for apair of rotor blades is also provided. However, in this embodiment,instead of employing a central needle-bearing type of joint with therotor shaft, the rotor shaft parts 51c and 51d are spaced verticallyfrom each other and are interconnected with a fitting having laterallyspaced flat or rectangular portions 51b (as in FIG. 8) providing atransverse channel through which the central blade mounting piece 50aextends.

Instead of employing a central roller bearing type of joint connectingthe blade mounting piece 50a with the rotor shaft, the embodiment ofFIGS. 9 and 10 includes a flexible joint of cylindrical form having amultiplicity of alternate layers of rubber and metal of constructionsimilar to that described above in connection with FIGS. 3 and 4. Thisjoint material is indicated in FIGS. 9 and 10 at 39a, and comprisesalternate layers of rubber and metal bonded to each other. The layers inthe joint of FIGS. 9 and 10 are cylindrical, rather than spherical as inFIGS. 3 to 7. This provides a yielding type of center joint about whicha portion of the flapping motion of the pair of rotor blades may occur,particularly concurrent flapping motions of the two blades in oppositesenses, i.e., one blade upwardly and the other blade downwardly. Thisjoint 39a is accommodated between the concave cylindrical fittings 35aand 36a in a manner similar to the arrangement described above inconnection with FIGS. 3 to 6.

As in the preceding embodiments, the arrangement of FIGS. 9 and 10 alsocontemplates the employment of the bellows components 42 fortransmitting driving torque from the rotor shaft 30 to the forked blademounting fittings 20a.

From the foregoing, it will be seen that the flexible bellows 42 may beemployed as a torque transmitting system between the rotor blades andthe rotor shaft in a variety of forms of equipment, in some of whichrelative angular or flapping motion of the blades in relation to therotor mounting shaft is permitted by employment of a yielding centraljoint, or in which, particularly in the case of a teetering type ofrotor embodying only a pair of blades, as in FIG. 8, relative angularmotion is permitted by employing the spherical type of roller bearing52a and by providing some clearance for relative angular motion of thecentral blade mounting piece 50 and the rotor shaft. Even where centralflexible or yielding joints are employed, as in the embodiments of FIGS.3 to 7, and FIGS. 9 and 10, additional freedom for relative angularmotion may be provided by appropriate clearance of mounting parts, inorder to assure that at least the primary torque force incident to thedriving of the rotor will be carried by the flexible bellows.

Turning now to FIGS. 11A and 11 B, it is first noted that these figuresare somewhat diagrammatic representations of certain actions of a rotorconstructed in accordance with the present invention (FIG. 11B) ascompared with a rotor of prior art type (FIG. 11A) in which the centraljoint described above, for instance, in connection with FIGS. 3 to 7, isnot employed. Thus, as shown in FIG. 11A, the arrangement requires thatflapping motions of the blades occur entirely by virtue of the flexstraps interconnecting the blades and the central hub structure, andthis configuration necessarily encounters a Coriolis effect, withresultant undesirable forces and vibrations introduced into the systemand communicated to the body of the aircraft.

Because of the presence of the central joint (FIG. 11B) as fullydescribed above, particularly in connection with FIGS. 3 to 7 inclusive,the rotor drive is of constant velocity, and the Coriolis effect issubstantially eliminated.

Further analysis of the preferred embodiment of rotor system, asdisclosed in the present application, is given in the table appearingherebelow, which gives comparative figures for three types of rotors.

In Table 1 herebe1ow, the three types of rotors are marked with theletters A, B, and C, and a brief identification of each rotor is givenin the table.

It will be seen that rotor A is identified as "Hingeless rotor", whichis a term which has been applied to this form of rotor, and this refersto a rotor of the type in which the flapping action is provided by theemployment of flex straps but in which pitch bearings are present inorder to provide for pitch change motion of the rotor blades.

Rotor B is identified as "Bearingless rotor", which is a term which hasbeen applied to this form of rotor, and this refers to a rotor of thetype in which both the flapping action and the pitch change action areprovided by the employment of flex straps.

The third type of rotor included in the table is identified as"Bearingless rotor with constant velocity joint and `soft hub`". In therotor here referred to, the flex straps are relied upon for bothflapping and pitch change action of the rotor blades, and this rotorfurther includes both the central constant velocity joint disclosed inthe present application, as well as the bellows driving or torqueconnection between the blades and the rotor hub, as is also disclosed inthe present application.

The rotors in any one of the three categories (A, B, or C) referred toin the table may also include pivots providing for lead-lag motion ofthe blades, or the flex straps employed may be constructed toaccommodate lead-lag motions, in addition to the other motions of theblades referred to.

                                      TABLE 1                                     __________________________________________________________________________                BLADE BLADE   VIRTUAL                                                         1ST FLAP.                                                                           1ST CHORD.                                                                            FLAPPING                                                        FREQ. FREQ.   HINGE  HUB MOMENT                                                                             AIRCRAFT                            TYPE OF ROTOR                                                                             PER REV.                                                                            PER REV.                                                                              LOCATION                                                                             (LB-IN) × 10.sup.3                                                               G.W.                                __________________________________________________________________________    A           1.12  .67      14.5% 70       5100 LBS.                           HINGELESS ROTOR                                                               B           1.048 .67       6.05%                                                                              35       5100 LBS.                           BEARINGLESS                                                                   ROTOR                                                                         C           1.021 .66     3%     18.31    5100 LBS.                           BEARINGLESS                                                                   ROTOR WITH                                                                    CONSTANT                                                                      VELOCITY JOINT                                                                AND "SOFT" HUB                                                                __________________________________________________________________________

As will be seen, Table 1 identifies certain characteristics of rotors inthe three categories identified, each aircraft having a gross weight of5100 lbs.

The values appearing in the column of the table referring to the "Blade1st Flap. Freq. Per Rev." are of special significance and determinevirtual flapping hinge location and also the hub moments. It isparticularly to be noted that with the rotor (C) of the presentapplication, the virtual flapping hinge is even shifted closer to thecenter of rotation, thereby still further reducing hub moments.

In comparing the hub moments with respect to rotors of the typesrepresented by A, B, and C in the above table, the following comparisonis of significance.

A. 70×10³ LB-IN - This represents very high vibration (1st Flap. Freq.1.12 cyc/rev-Virtual Hinge Location 14.5%).

B. 35×10³ LB-IN - This represents moderate vibration (1st Flap. Freq.1.048 cyc/rev-Virtual Hinge Location 6.05%).

C. 18.31×10³ LB-IN - This represents very low vibration (1st Flap. Freq.1.021 cyc/rev Virtual Hinge Location 3%).

From the foregoing, it will be seen that the arrangement of the presentinvention is capable of providing rotor operation with greatly reducedvibration characteristics.

I claim:
 1. A bladed aircraft rotor comprising a rotative hub structureand two blades radiating from the hub structure, the hub structureincluding a driving shaft, a blade mounting device, each blade having amounting strap for connecting the blade with said mounting device andproviding freedom for flapping motions of the blade with respect to themounting device, means connecting the mounting device with the driveshaft with freedom for motion in a direction in the blade flappingplane, and means for transmitting torque forces between the hubstructure and the blades comprising flexible bellows above and below theblade mounting device for interconnecting the blade mounting device withthe drive shaft above and below the blade mounting device.
 2. A bladedaircraft rotor as defined in claim 1, in which the means connecting theblade mounting device with the drive shaft comprises a bearing having ahorizontal axis intersecting the axis of rotation of the driving shaftand perpendicular to the axis of the connected blades.
 3. A bladedaircraft rotor comprising a rotative hub structure and two bladesradiating from the hub structure, the hub structure including a drivingshaft, a blade mounting device, each blade having a mounting strap forconnecting the blade with said mounting device and providing freedom forflapping motions of the blade with respect to the mounting device, meansconnecting the mounting device with the drive shaft with freedom formotion in a direction in the blade flapping plane, said means comprisinga joint providing for teetering of the rotor blades in the flappingplane and also for concurrent movement of the blades in the lead-lagsense, and means for transmitting torque forces between the drivingshaft and the blade mounting device comprising an annular sheetcomponent having one edge connected with the driving shaft and the otheredge connected with the blade mounting device, said sheet componentbeing transversely flexible to accommodate relative angular motion ofthe blade mounting device and the drive shaft.
 4. A bladed aircraftrotor as defined in claim 3, in which said joint comprises a sphericalroller bearing.
 5. A bladed aircraft rotor as defined in claim 4, inwhich said joint comprises a laminated assembly of flexible metallic andelastomeric sheet materials.